High-Resolution X-Ray Diffraction Measurement with Enhanced Sensitivity

ABSTRACT

A method for analysis includes directing a converging beam of X-rays toward a surface of a sample having an epitaxial layer formed thereon, and sensing the X-rays that are diffracted from the sample while resolving the sensed X-rays as a function of angle so as to generate a diffraction spectrum including a diffraction peak and fringes due to the epitaxial layer. A characteristic of the fringes is analyzed in order to measure a relaxation of the epitaxial layer.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application is a continuation of U.S. patentapplication Ser. No. 12/683,436, filed Jan. 7, 2010, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to X-ray analysis, andspecifically to X-ray measurement of thin film properties.

BACKGROUND OF THE INVENTION

X-ray diffractometry (XRD) is a well-known technique for studying thecrystalline structure of matter. In XRD, a sample is irradiated by amonochromatic X-ray beam, and the locations and intensities of thediffraction peaks are measured. The characteristic diffraction anglesand the intensity of the diffracted radiation depend on the latticeplanes of the sample under study and the atoms that occupy those planes.For a given wavelength λ and lattice plane spacing d, diffraction peakswill be observed when the X-ray beam is incident on a lattice plane atangles θ that satisfy the Bragg condition: nλ=2d sin θ, wherein n is thescattering order. The angle θ that satisfies the Bragg condition isknown as the Bragg angle. Distortions in the lattice planes due tostress, solid solution, or other effects lead to observable changes inthe XRD spectrum.

XRD has been used, inter alia, for measuring characteristics ofepitaxial films produced on semiconductor wafers. For example, Bowen etal. describe a method for measuring germanium concentration in a SiGestructure using high-resolution XRD in “X-Ray metrology by Diffractionand Reflectivity,” Characterization and Metrology for ULSI Technology,2000 International Conference (American Institute of Physics, 2001),which is incorporated herein by reference.

XRD may also be used at grazing incidence to observe structures on thesurface of a sample. For example, Goorsky et al. describe the use ofgrazing-incidence XRD for analyzing epitaxial layer structures on asemiconductor wafer in “Grazing Incidence In-plane DiffractionMeasurement of In-plane Mosaic with Microfocus X-ray Tubes,” CrystalResearch and Technology 37:7 (2002), pages 645-653, which isincorporated herein by reference. The authors apply the technique todetermine the in-plane lattice parameter and lattice orientation of verythin surface and buried semiconductor layers.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide methods and systems that enhance the sensitivity and accuracy ofhigh-resolution XRD measurements. These methods and systems are usefulparticularly in measuring features of epitaxial thin-film layers, butthey may also be applied in analyzing crystalline structures of othertypes.

There is therefore provided, in accordance with an embodiment of thepresent invention, a method for analysis, including directing aconverging beam of X-rays toward a surface of a sample having first andsecond crystalline layers, with different, respective crystalcharacteristics. The X-rays that are diffracted from the sample aresensed while resolving the sensed X-rays as a function of angle so as togenerate a first diffraction spectrum including at least a firstdiffraction peak due to the first layer and a second diffraction peakdue to the second layer. A beam blocker is positioned in the convergingbeam so as to block a range of angles containing the first diffractionpeak, and the X-rays that are diffracted from the sample while the beamblocker is positioned in the converging beam are sensed so as togenerate a second diffraction spectrum including at least the seconddiffraction peak while the first diffraction peak at least partlyblocked. At least the second diffraction spectrum is analyzed so as toidentify a characteristic of at least the second layer.

In a disclosed embodiment, sensing the X-rays includes deploying adetector array having elements configured to capture and resolve theX-rays over a range of elevation angles simultaneously, wherein therange is at least 2 degrees.

Typically, the second layer is deposited epitaxially over the firstlayer. In a disclosed embodiment, the first layer includes asemiconductor substrate, such as a silicon wafer, and the second layerincludes a doped semiconductor, such as a SiGe epitaxial layer.

In one embodiment, analyzing at least the second diffraction spectrumincludes analyzing a fringe pattern appearing in a vicinity of the firstdiffraction peak in the second diffraction spectrum.

Positioning the beam blocker may include automatically analyzing thefirst diffraction spectrum so as to identify an angular range of thefirst diffraction peak, and automatically shifting the blocker to coverthe identified range.

In some embodiments, the converging beam of X-rays has a focus, andsensing the X-rays to generate the first diffraction spectrum includesshifting the sample out of the focus so as to increase a separationbetween the first and second diffraction peaks. Positioning the beamblocker includes adjusting a position of beam blocker while the sampleis out of the focus, and then shifting the sample into the focus inorder to generate the second diffraction spectrum. Sensing the X-rays togenerate the first diffraction spectrum may include shifting the sampleout of the focus and capturing at least the first diffraction spectrumin an asymmetric diffraction mode.

There is also provided, in accordance with an embodiment of the presentinvention, a method for analysis, including directing a converging beamof X-rays, having a focus, toward a surface of a sample having first andsecond crystalline layers, with different, respective crystalcharacteristics. The X-rays that are diffracted from the sample aresensed while resolving the sensed X-rays as a function of angle so as togenerate a diffraction spectrum including at least a first diffractionpeak due to the first layer and a second diffraction peak due to thesecond layer. The sample is shifted out of the focus so as to increase aseparation between the first and second diffraction peaks in thediffraction spectrum, and the diffraction spectrum is analyzed so as toidentify a characteristic of at least the second layer.

In some embodiments, the X-rays in the converging beam impinge on thesample over a range of incidence angles, and sensing the X-rays includesdetecting the X-rays in an asymmetric mode, in which the X-rays arediffracted from the sample at takeoff angles that are different from theincidence angles. In one embodiment, the second layer is depositedepitaxially over the first layer, and analyzing the diffraction spectrumincludes detecting a relaxation of the second layer relative to thefirst layer. The converging beam of the X-rays impinges on a spot on thesurface of the sample, and the method, alternatively or additionally,includes positioning a beam limiter to block a portion of the X-rays ina location adjacent to the spot so as to reduce a dimension of the spot.

In a disclosed embodiment, shifting the sample includes measuring theseparation as a function of a distance of the sample from the focus.Analyzing the diffraction spectrum may include finding a concentrationof a dopant in the second layer based on a functional dependence of theseparation on the distance of the sample from the focus.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for analysis, including directing aconverging beam of X-rays to impinge over a range of incidence angles ona spot on a surface of a sample having an epitaxial layer formedthereon. A beam limiter is positioned to block a portion of the X-raysin a location adjacent to the spot so as to reduce a dimension of thespot. The X-rays that are diffracted from the spot are sensed in anasymmetric mode, in which the X-rays are diffracted from the sample attakeoff angles that are different from the incidence angles, whileresolving the sensed X-rays as a function of angle so as to generate adiffraction spectrum. The diffraction spectrum is analyzed in order toidentify a characteristic of the epitaxial layer.

In a disclosed embodiment, the beam limiter includes a knife edge, whichis positioned parallel to the surface over the spot. Alternatively, thebeam limiter has a hole configured for passage of the X-raystherethrough, such that the dimension of the spot is determined by asize of the hole.

Typically, either the incidence angles or the takeoff angles fall withina range of grazing angles.

There is further provided, in accordance with an embodiment of thepresent invention, a method for analysis, including directing aconverging beam of X-rays toward a surface of a sample having anepitaxial layer formed thereon. The X-rays that are diffracted from thesample are sensed while resolving the sensed X-rays as a function ofangle so as to generate a diffraction spectrum including a diffractionpeak and fringes due to the epitaxial layer. A characteristic of thefringes is analyzed in order to measure a relaxation of the epitaxiallayer.

In a disclosed embodiment, analyzing the characteristic includesassessing an amplitude of the fringes, wherein a reduction in theamplitude is indicative of an increase in the relaxation.

In some embodiments, the sample includes a crystalline substrate, anddirecting the converging beam includes positioning a beam blocker in theconverging beam so as to block a range of angles containing a substratediffraction peak while enhancing detection of the fringes at anglesadjacent to the range that is blocked.

There is moreover provided, in accordance with an embodiment of thepresent invention, a method for analysis, including directing aconverging monochromatic first beam of X-rays at a first wavelengthtoward a focus on a surface of a crystalline sample. A second, undesiredbeam at a second wavelength is blocked at a location adjacent to thefirst beam and before the focus. The X-rays that are diffracted from thesample are sensed while resolving the sensed X-rays as a function ofangle so as to generate a diffraction spectrum of the sample. Thediffraction spectrum is analyzed so as to identify a characteristic ofthe sample.

In a disclosed embodiment, directing the first beam includes focusing aninput X-ray beam using a curved crystal monochromator, which alsogenerates the second beam.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for analysis, including directing aconverging beam of X-rays toward a focus on a surface of a crystallinesample. A slit is scanned across the converging beam so as to cause theX-rays in the beam to be incident on the sample at a sequence of anglesof incidence over an angular range of the beam. At each of the angles ofincidence, the X-rays that are diffracted from the sample are sensedwhile resolving the sensed X-rays as a function of takeoff angle so asto generate diffraction data with respect to each of the angles ofincidence. The diffraction data with respect to the angles of incidenceare combined over the angular range so as to generate a reciprocal spacemap of diffraction from the sample.

There is moreover provided, in accordance with an embodiment of thepresent invention, apparatus for analysis, including an X-ray source,which is configured to direct a converging beam of X-rays toward asurface of a sample having first and second crystalline layers, withdifferent, respective crystal characteristics, whereby the X-rays arediffracted from the sample so as to generate a diffraction spectrumincluding at least a first diffraction peak due to the first layer and asecond diffraction peak due to the second layer. A detector assembly isconfigured to sense the X-rays that are diffracted from the sample whileresolving the sensed X-rays as a function of angle. A beam blocker isconfigured to be positioned in the converging beam so as to block arange of angles containing the first diffraction peak. A processor iscoupled to receive and process an output of the detector assembly, whilethe range of the angles containing the first diffraction peak isblocked, so as to identify a characteristic of at least the second layerbased on the diffraction spectrum.

There is furthermore provided, in accordance with an embodiment of thepresent invention, apparatus for analysis, including an X-ray source,which is configured to direct a converging beam of X-rays toward asurface of a sample having first and second crystalline layers, withdifferent, respective crystal characteristics, whereby the X-rays arediffracted from the sample so as to generate a diffraction spectrumincluding at least a first diffraction peak due to the first layer and asecond diffraction peak due to the second layer. A detector assembly isconfigured to sense the X-rays that are diffracted from the sample whileresolving the sensed X-rays as a function of angle. A motion device isconfigured to shift the sample out of the focus so as to increase aseparation between the first and second diffraction peaks in thediffraction spectrum. A processor is coupled to receive and process anoutput of the detector assembly, while the sample is shifted out of thefocus, so as to identify a characteristic of at least the second layerbased on the diffraction spectrum.

There is also provided, in accordance with an embodiment of the presentinvention, apparatus for analysis, including an X-ray source, which isconfigured to direct a converging beam of X-rays to impinge over a rangeof incidence angles on a spot on a surface of a sample having anepitaxial layer formed thereon, whereby the X-rays are diffracted fromthe sample so as to generate a diffraction spectrum. A beam limiter isconfigured to block a portion of the X-rays in a location adjacent tothe spot so as to reduce a dimension of the spot. A detector assembly isconfigured to sense the X-rays that are diffracted from the spot in anasymmetric mode, in which the X-rays are diffracted from the sample attakeoff angles that are different from the incidence angles, whileresolving the sensed X-rays as a function of angle. A processor iscoupled to receive and process an output of the detector assembly so asto identify a characteristic of the epitaxial layer based on thediffraction spectrum.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for analysis, including an X-ray source,which is configured to direct a converging beam of X-rays toward asurface of a sample having an epitaxial layer formed thereon, wherebythe X-rays are diffracted from the sample so as to generate adiffraction spectrum including a diffraction peak and fringes due to theepitaxial layer. A detector assembly is configured to sense the X-raysthat are diffracted from the sample while resolving the sensed X-rays asa function of angle. A processor is coupled to receive and process anoutput of the detector assembly so as to measure a relaxation of theepitaxial layer based on a characteristic of the fringes.

There is further provided, in accordance with an embodiment of thepresent invention, apparatus for analysis, including an X-ray source,which is configured to direct a converging monochromatic first beam ofX-rays at a first wavelength toward a focus on a surface of acrystalline sample. A beam blocker is configured to be positioned so asto block a second beam at a second wavelength at a location adjacent tothe first beam and before the focus. A detector assembly is configuredto sense the X-rays that are diffracted from the sample while resolvingthe sensed X-rays as a function of angle so as to generate a diffractionspectrum of the crystalline sample. A processor is coupled to receiveand process the diffraction spectrum so as to identify a characteristicof the sample.

There is moreover provided, in accordance with an embodiment of thepresent invention, apparatus for analysis, including an X-ray source,which is configured to direct a converging beam of X-rays toward a focuson a surface of a crystalline sample. A slit is configured to scanacross the converging beam so as to cause the X-rays in the beam to beincident on the sample at a sequence of angles of incidence over anangular range of the beam. A detector assembly is configured to sensethe X-rays that are diffracted from the sample at each of the angles ofincidence, while resolving the sensed X-rays as a function of takeoffangle so as to generate diffraction data with respect to each of theangles of incidence. A processor is configured to combine thediffraction data with respect to the angles of incidence over theangular range so as to generate a reciprocal space map of diffractionfrom the sample.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a system for high-resolution X-raydiffraction (HRXRD) measurement, in accordance with an embodiment of thepresent invention;

FIG. 2 is a schematic side view of the system of FIG. 1 in analternative configuration, in accordance with an embodiment of thepresent invention;

FIG. 3A is a schematic, pictorial illustration of elements of a HRXRDsystem, showing the use of a beam blocker in accordance with anembodiment of the present invention;

FIG. 3B is a schematic top view of elements of a HRXRD system, showingthe use of a beam blocker in accordance with another embodiment of thepresent invention;

FIG. 4 is a plot that schematically shows HRXRD spectra taken atdifferent positions of a beam blocker, in accordance with an embodimentof the present invention;

FIGS. 5A and 5B are schematic, sectional views of an epitaxial layer ona substrate in pseudomorphic and relaxed configurations, respectively;

FIG. 6 is a plot that schematically shows HRXRD spectra measured inaccordance with an embodiment of the present invention, at differentdegrees of relaxation of an epitaxial layer;

FIG. 7 is a schematic top view of a sample showing incidence of an X-raybeam on the sample in different beam-spread conditions, in accordancewith an embodiment of the present invention;

FIG. 8 is a schematic, pictorial illustration of elements of a HDXRDsystem, showing the use of a beam limiter to control beam spread inaccordance with an embodiment of the present invention;

FIG. 9 is a schematic, sectional view of a beam limiter, in accordancewith another embodiment of the present invention;

FIG. 10 is a plot that schematically shows a dependence ofcharacteristics of a HRXRD spectrum on sample height, in accordance withan embodiment of the present invention; and

FIG. 11 is a schematic side view of a system for HRXRD measurement, inaccordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Embodiments of the present invention that are described hereinbelowprovide enhanced methods and systems for analysis of crystalline samplesusing high-resolution X-ray diffraction (HRXRD). The disclosedembodiments afford enhanced accuracy in characterizing thin-filmepitaxial layers, and are thus particularly useful in testing andmonitoring the production of semiconductor devices. The principles ofthe present invention, however, may similarly be applied in studying andcharacterizing samples of other kinds.

In the disclosed embodiments, a converging beam of X-rays is directedtoward a surface of a sample, which typically includes multiplecrystalline layers (for example, a silicon substrate with an epitaxialdoped layer, such as SiGe, formed on the surface). A detector assembly,which typically comprises a detector array, senses the X-rays that arediffracted from the sample while resolving the sensed X-rays as afunction of angle. The detector assembly thus captures a diffractionspectrum, which typically includes a respective diffraction peak due toeach of the layers, and possibly weaker features, as well, such as afringe pattern. The separation between the peaks is often indicative ofthe composition of the layers, such as the concentration of thegermanium dopant in the SiGe layer. The fringe pattern can provideinformation on physical dimensions of the crystalline layer structure.

The diffraction peak due to one or the layers, such as the peak arisingfrom the substrate of the sample, is often strong and may tend to hideor wash out the weaker features. Therefore, in some embodiments, a beamblocker is introduced into the converging beam so as to block a range ofangles containing the strong diffraction peak. The resulting diffractionspectrum permits the weaker features, such as the above-mentioned fringepattern, to be measured with greater accuracy. Methods for facilitatingaccurate placement of the beam blocker are described hereinbelow.

Relaxation of epitaxial layers, in which the crystalline structure of athin-film layer comes out of alignment with the substrate (or otherlayer) below it, can cause defects in semiconductor devices containingthese layers. It is therefore important to detect relaxation and to makeprocess adjustments, if required, to reduce relaxation insubsequently-manufactured wafers. In some embodiments of the presentinvention, grazing-angle asymmetric HRXRD is used to measure relaxation.In another embodiment, the amplitude of the fringes in the diffractionspectrum provides a measure of relaxation. Typically, a reduction in thefringe amplitude is indicative of an increase in the relaxation.

Normally, the X-ray source and sample are positioned so that theconverging beam of X-rays is focused to a spot on the sample. Theinventors have discovered, however, that shifting the sample out of thefocus when operating in asymmetric mode can increase the separationbetween the peaks in the diffraction spectrum. (In asymmetric mode, theX-rays are diffracted from the sample at takeoff angles that aredifferent from the incidence angles, in contrast to the symmetric mode,in which the incidence and takeoff angles are the same.) Therefore, insome embodiments, the sample is intentionally positioned out of focus,in order to permit more accurate measurement of secondary peaks andother weak features in the spectrum. Furthermore, the relative shiftbetween the peaks as a function of the distance of the sample from thefocus can be measured to provide useful information regarding anepitaxial layer on the sample, such as the concentration of a dopant inthe layer.

Additionally or alternatively, while the sample is out of focus and thediffraction peaks are far apart, a beam blocker can be preciselypositioned to block a strong peak, such as the substrate peak, afterwhich the sample may be shifted back into focus.

In asymmetric mode, the X-ray beam may be incident on the sample at agrazing angle, or the diffracted X-ray beam may be detected at a grazingangle. The term “grazing angle,” in the context of the present patentapplication, means an angle that is close to the surface of the sample,typically within 10° of the surface. Grazing angle measurements areuseful in detecting diffraction from crystal planes that are notparallel to the sample surface and may be used, for example, to measurerelaxation of an epitaxial layer. They have the disadvantage, however,that the spot area from which diffraction is detected is elongated alongthe beam axis, and diffraction measurements made on small features mayconsequently be distorted. To alleviate this problem, in someembodiments of the present invention, a beam limiter is positioned toblock a portion of the X-rays in a location adjacent to the spot on thesample. The beam limiter reduces the dimension of the spot along thebeam axis direction and thus can improve measurement accuracy.

System Description

FIG. 1 is a schematic side view of a system 20 for HRXRD of a sample 22,in accordance with an embodiment of the present invention. In theembodiments that are described hereinbelow, sample 22 is taken to be asilicon wafer on which an epitaxial layer is deposited, and the HRXRDcapabilities of system 20 are applied in analyzing characteristics ofthe epitaxial layer. In alternative embodiments, however, system 20 maybe used to analyze crystalline samples of other types. Additionally oralternatively, system 20 may be configured to carry out other types ofX-ray measurements, such as measurements of X-ray reflectometry (XRR),X-ray fluorescence (XRF), and small-angle X-ray scattering (SAXS), asdescribed, for example, in U.S. Pat. Nos. 7,120,228 and 7,551,719, whosedisclosures are incorporated herein by reference.

In system 20, sample 22 is mounted on a motion device, such as a motionstage 24, allowing accurate adjustment of the position and orientationof the sample. Alternatively or additionally, the motion device mayshift and adjust other elements of the system relative to the sample. AnX-ray source 26 directs a converging X-ray beam 30 onto a spot 32 onsample 22. Generally, source 26 and stage 24 are adjusted so that thefocus of beam 30 is located precisely at spot 32 on the sample surface,but in some cases (as described in greater detail hereinbelow), thesample height (Z-coordinate) may be shifted out of the beam focus. Adetector assembly 36 detects a diverging beam 34 of X-rays that isdiffracted from the sample.

Typically, source 26 comprises an X-ray tube 38 with suitable optics 40to focus and monochromatize beam 30. Beam 30 typically subtends at least2°, and may subtend as much as 4° or even more, depending on optics 40,in order to irradiate sample 22 over a large range of anglessimultaneously. Optics 40 may comprise, for instance, a curved crystalmonochromator, which focuses and monochromatizes an input beam from tube38. Further details of X-ray tubes and optics that may be used in thiscontext are described, for example, in the above-mentioned U.S. Pat.Nos. 7,120,228 and 7,551,719, as well as in U.S. Pat. No. 7,076,024,whose disclosure is incorporated herein by reference.

Detector assembly 36 typically comprises a detector array 42, such as aCCD array, comprising multiple detector elements, configured so as toresolve beam 34 as a function of elevation angle θ. Detector assembliesof this type are also described in the above-mentioned patents.Typically, the angular span of array 42 is comparable to that of beam30, i.e., at least 2°, and possibly 4° or greater. A beam blocker 48 anda beam limiter 50 (such as a knife edge) and/or other optical elementsmay be used to limit beam 30 and/or beam 34 and to block undesiredscattered radiation that might otherwise strike array 42 and interferewith the diffraction measurement. Another beam blocker 49, orientedperpendicularly to blocker 48, is used to block undesired irradiationwavelengths. The use of these elements in enhancing HRXRD measurementsis described in greater detail hereinbelow.

The positions of source 26 and detector assembly 36 are controlled bymotion assemblies 44 and 46, respectively. In the simplified view shownin FIG. 1, the motion assemblies comprise curved tracks, which permitthe source and detector assembly to be positioned at the appropriateelevations, typically in the vicinity of the Bragg angles of the layersthat are to be analyzed. Other suitable motion assemblies mayalternatively be used, as will be apparent to those skilled in the art.For the sake of the example shown in FIG. 1, it is assumed that thelattice planes creating the diffraction pattern of interest areapproximately parallel to the surface of sample 22, so that theincidence and takeoff angles defined by beams 30 and 34 relative to thesurface are both equal to the Bragg angle. (This assumption is oftentrue with respect to semiconductor substrates, such as silicon wafers,and epitaxial layers that are grown on such substrates.) Alternatively,source 26 and detector assembly 36 may be positioned at differentincidence and takeoff angles, as shown in FIG. 2, for example, in orderto measure diffraction from lattice planes that are not parallel to thesurface of sample 22.

In a typical embodiment, as noted above, stage 24 is configured totranslate the height (Z-coordinate) of sample 22 and the X-Y location onthe sample that falls within spot 32, as well as to rotate the azimuthalangle Φ and incidence angle of the sample relative to beam 30. (As shownin FIG. 1, the X-Y plane is taken to be the sample surface, with theZ-axis perpendicular to the surface; θ is the elevation angle relativeto the Z-axis; and Φ is the azimuthal angle of rotation about theZ-axis.) Additionally or alternatively, these position and angleadjustments may be achieved by moving or otherwise adjusting otherelements of system 20, such as the source and detector assemblies.

A signal processor 52 receives and analyzes the output of assembly 36,so as to measure a spectrum 54 of the flux of X-ray photons diffractedfrom sample 22 as a function of elevation angle θ at a given energy orover a range of energies. Typically, spectrum 54 as a function ofelevation angle exhibits a structure that is characteristic ofdiffraction effects due to the surface layer and underlying layers,including the sample substrate. Processor 52 analyzes the angularspectrum in order to identify characteristics of one or more of thelayers of the sample, such as the composition, lattice strain (orequivalently, relaxation) and/or tilt angle of a given layer, usingmethods of analysis described hereinbelow.

As noted above, the components of system 20 and the techniques describedherein may be used to provide other types of measurement functionality,such as X-ray reflectometry and scattering measurements. Additionally oralternatively, these components and techniques may be integrated asprocess monitoring tools in manufacturing systems, such as systems forsemiconductor wafer fabrication. For example, in an alternativeembodiment of the present invention (not shown in the figures), elementsof system 20 are integrated with a semiconductor wafer fabrication toolto provide in situ inspection. Typically, the fabrication tool comprisesa vacuum chamber containing deposition apparatus for creating thin filmson a wafer, as is known in the art. The chamber has X-ray windows, asdescribed, for instance, in U.S. Patent Application Publication US2001/0043668, now U.S. Pat. No. 6,970,532, whose disclosure isincorporated herein by reference. X-ray source assembly 26 may thenirradiate spot 32 on the wafer via one of the windows, and detectorassembly 36 may receive the scattered X-rays through another window. Inanother alternative embodiment, system 20 may be configured as a stationin a cluster tool, along with other stations used in performingproduction steps.

FIG. 2 is a schematic side view of system 20 in an alternative,asymmetric configuration, in accordance with an embodiment of thepresent invention. In this case, source 26 is positioned by motionassembly 44 so as to irradiate sample 22 at a grazing angle, with thecentral beam axis 8° from the sample surface, for example. Motionassembly 46 positions detector assembly 36 at a high angle, for example,79°, in order to capture Bragg diffraction from lattice planes that arenot parallel to the sample surface. This configuration is useful inmeasuring the spacing between cells of the crystal lattice along thedirection parallel to the sample surface and can thus be used inassessing relaxation of epitaxial layers (as illustrated below in FIGS.5A and 5B).

In an alternative embodiment, not shown in the figures, beam 30 mayirradiate the sample surface at a high angle, while detector assembly 36is positioned to capture X-rays diffracted from the sample at grazingangles.

Enhancement of Resolution by Use of Beam Blockers

FIG. 3A is a schematic, pictorial illustration of elements of HRXRDsystem 20, showing the use of beam blocker 48 in accordance with anembodiment of the present invention. In the default position, the beamblocker is retracted, as shown in FIG. 1, and does not impinge onincident beam 30. In some circumstances, however, it is advantageous toblock a certain angular range within the beam. Beam blocker 48 may bepositioned to block an upper portion of the range, as illustrated inFIG. 3A, or it may alternatively be positioned to block a lower portion.The corresponding range of angles in diffracted beam 34 will similarlybe cut off, or at least attenuated. This application of the beam blockeris useful in attenuating intense components of the diffraction spectrum,in order to reduce the dynamic range of the diffracted beam andfacilitate detection of weak features that might otherwise be washedout.

FIG. 3B is a schematic top view of elements of HDXRD system 20, showingthe use of beam blocker 49 in accordance with an embodiment of thepresent invention. X-ray tube 38 typically emits multiple X-raywavelengths, which may be closely spaced, such as the Cu Ka1 and Ka2wavelengths. Optics 40 (shown in this figure as a curved crystalmonochromator) may not fully filter out nearby emission lines. Thus, inthe present example, when beam 30 comprises the Cu Ka1 line, optics 40also focus the Cu Ka2 line from tube 38 into an adjacent beam 56. In atypical configuration of system 20, the focus of beam 56 is onlyslightly displaced, by less than 1 mm, and in some cases less than 0.2mm, from focal spot 32 of beam 30. (The separation between the beams isexaggerated in the figure for the sake of clarity.) Scattered Cu Ka2radiation may therefore reach detector assembly 36 and interfere withthe HDXRD measurement.

Beam blocker 49 can be used to alleviate this problem. This beam blockermay comprise, for example, a metal knife edge oriented in the vertical(Z) direction. The knife edge is adjusted, as shown in FIG. 3B, to blockbeam 56 at a location a few millimeters before the focus. Beam 56 iswell separated from beam 30 at this location, and beam blocker 49 thusdoes not intercept the desired beam 30.

FIG. 4 is a plot 60 that schematically shows HRXRD spectra taken atdifferent positions of beam blocker 48, in accordance with an embodimentof the present invention. An unblocked spectrum 62, with the beamblocker withdrawn from beam 30, is dominated by a substrate peak 64, dueto Bragg diffraction from the silicon substrate of the wafer under test.In this example, an epitaxial SiGe layer has been formed on thesubstrate, and spectrum 62 includes a side peak 66 due to Braggdiffraction from the SiGe layer. (The angular separation between peaks64 and 66 is indicative of the concentration of the germanium dopant inthe SiGe.) A fringe structure 70 in the intermediate angular regionbetween peaks 64 and 66, however, is difficult to see in spectrum 62because of the spread of radiation from peak 64 into this region.

To alleviate this problem, once the location of peak 64 has beenascertained in spectrum 62, beam blocker 48 is positioned to block thecorresponding range of angles in incident beam 30. The beam blocker ispositioned precisely in order to minimize blockage of the of theintermediate region that contains fringe structure 70. In a resultingblocked spectrum 68 in FIG. 4, peak 64 is largely suppressed, and theresolution of the fringe structure is therefore enhanced. The period andamplitude of this fringe structure can provide valuable informationregarding the dimensions of the epitaxial layer.

In one embodiment, processor 52 controls beam blocker 48 on the basis ofspectrum 62. The processor analyzes the spectrum in order to find thelocation and width of peak 64. The processor then computes the desiredposition of the beam blocker in order to attenuate peak 64 and outputs acontrol signal to the beam blocker accordingly. The beam blockertypically comprises a motion control device, such as a motor with linearencoder, which is actuated to position the beam blocker according to thesignal from processor 52.

Although the use of beam blocker 48 is illustrated in FIGS. 1 and 3A inthe symmetric diffraction mode, the beam blocker may be used in likemanner for the same purpose in the asymmetric mode that is shown in FIG.2.

Measurement of Relaxation of Epitaxial Layers

FIGS. 5A and 5B are schematic, sectional views of samples 72 and 78,respectively, in which an epitaxial layer 76 has been formed on asubstrate 74. Layer 76 may comprise, for example, a thin film of SiGethat is formed on a silicon substrate. The addition of the germaniumdopant causes unit cells 77 in layer 76 to be larger in volume than theunit cells of substrate 74. (The difference is exaggerated in thefigures for the sake of visual clarity.) In sample 72, layer 76 ispseudomorphic, meaning that the unit cells in layer 76 are strained soas to maintain alignment with the underlying cells of layer 74. Insample 78, however, cells 77 in layer 76 have relaxed to a cubicconfiguration and have lost alignment with the underlying cells. Thissort of relaxation, which may result from improper settings in themanufacturing process, can cause defects in semiconductor devices thatare made from this wafer. It is therefore important to monitorrelaxation of epitaxial layers and to adjust the process appropriatelywhen relaxation is detected.

One method for monitoring relaxation is to measure changes in therelative positions of the diffraction peaks dues to the substrate and tothe layer in question in asymmetric measurement mode. An alternativemethod, which may provide more accurate results, is to analyze thefringe structure in the diffraction spectrum.

FIG. 6 is a plot that schematically shows HRXRD spectra 80, 82 and 84,illustrating the effect of relaxation of an epitaxial layer on thefringe structure, in accordance with an embodiment of the presentinvention. The spectra were taken in symmetric mode from SiGe layersformed on a silicon substrate. The strong peaks due to the substratehave been blocked out of these spectra, as explained above.

Spectrum 80 was taken from a fully-strained SiGe layer. The fringesbetween the angles of about −0.55° and −0.85° are clearly visible andhave a large amplitude, on the order of 100 counts. On the other hand,there are no visible fringes at all in spectrum 84, which was taken froma fully-relaxed layer (with a dimensional shift of approximately 6.6%between unit cells of the SiGe layer and those of the underlyingsilicon). In the intermediate example of spectrum 82, the SiGe layer ismildly relaxed (dimensional shift of approximately 3%), and the fringesare visible but with much-reduced amplitude.

Based on these findings, processor 52 may analyze fringe amplitudes inHRXRD spectra in order to estimate the extent of relaxation of epitaxiallayers. The amplitude of the fringes may be extracted by parametricfitting of the spectrum to a model, and the resulting fit parameterswill give an accurate indication of the layer relaxation. The period ofthe fringes is indicative of the thickness of the epitaxial layer.

Enhancing Detection Accuracy in Asymmetric Mode

Reference is now made to FIGS. 7 and 8, which schematically illustratethe use of beam limiter 50 in asymmetric-mode HRXRD in system 20, inaccordance with an embodiment of the present invention. FIG. 7 is anenlarged top view of sample 22, showing spot 32 formed by X-ray beam 30on the sample and the effect of beam limiter 50 on the extent of thespot. FIG. 8 is a pictorial illustration of elements of system 20showing how beam limiter 50 (an adjustable knife edge in thisembodiment) is inserted into beam 30 in order to control the extent ofthe spot on the sample. Although FIG. 8 shows a configuration in whichincident beam 30 impinges on sample 22 at grazing incidence, the beamlimiter may similarly be used in the alternative configuration in whichthe diffracted beam takes off from the sample at a grazing angle. Theproblem addressed by the beam limiter is less acute in this alternativeconfiguration, but it may still be desirable to limit the extent of thespot.

In FIG. 7, an epitaxial layer is formed as a small pad 90 on the surfaceof sample 22. Because of the low incidence angle of beam 30, spot 32extends over pad 90 and also covers a large area of the substrate thatis not covered by the epitaxial layer. The mismatch between spot 32 andpad 90 has at least two undesirable consequences:

-   -   1) The strong peak in the HRXRD spectrum due to the substrate        (peak 64 in FIG. 4) will be enhanced relative to peak 66 and to        other spectral structure originating from the epitaxial layer.    -   2) The displacement of pad 90 relative to the center of spot 32        on sample 22 will distort the apparent angular separation        between layer peak 66 and substrate peak 64 in the XRD spectrum.

To alleviate these problems, beam limiter 50 is inserted into beam 30above spot 32. When a knife edge is used for beam limiting, for example,the knife edge is lowered to a small distance above the surface ofsample 22, so as to block the upper portion of beam 30 and also to blockdiffracted rays resulting from the lower portion of beam 30, asillustrated in the inset in FIG. 8. In a typical configuration, theknife edge is positioned on the order of 15 μm above the sample surface,but larger or smaller distances may be used depending on applicationrequirements. As a result of the beam limiter, the effective size of theirradiating beam is reduced, so that detector assembly 36 receivesdiffracted X-rays only from a reduced spot 92. The dimension of spot 92in this example is reduced in the Y-direction (the direction of theprojection of the axis of beam 30 onto the sample surface), so that spot92 falls almost entirely on pad 90. As a result, the relative strengthof layer peak 66 is enhanced in the diffraction spectrum, and thedistortion in the separation between substrate peak 64 and layer peak 66is eliminated.

FIG. 9 is a schematic, sectional view of a beam limiter 94, inaccordance with another embodiment of the present invention. The beamlimiter in this embodiment may be made, for example, from a metal rodhaving a cylindrical profile, as shown in the figure, or any othersuitable profile. A hole 96 through the rod reduces the effective sizeof spot 32 by geometrically limiting the area from which diffractedX-rays in beam 34 are allowed to reach the detector assembly. Incidentbeam 30 impinges on sample 22 below beam limiter 94 at a grazing angle,such as 8°, while diffracted beam 34 leaves the sample at about 79°through hole 96. The beam limiter may be about 0.4 mm in diameter, witha hole having a diameter of about 40 μm, but larger or smaller holes maybe used depending on the desired spatial resolution.

Beam limiter 94 may likewise be used in asymmetric grazing exit mode, inwhich sample 22 is irradiated at a high angle and the diffracted beam isdetected at grazing angles. In this case, the beam limiter is placed inincident beam 30, so that the incident X-rays pass through hole 96, thusdefining and limiting the spot from which the X-rays are diffracted.

FIG. 10 is a plot that schematically shows a dependence ofcharacteristics of a HRXRD spectrum on sample elevation, in accordancewith an embodiment of the present invention. Data points 98 in this plotcorrespond to the separation between substrate peak 64 and layer peak 66(as shown in FIG. 4) due to an epitaxial layer of SiGe, measured inasymmetric diffraction mode as a function of the height (Z-coordinate)of the silicon wafer. The Z-coordinate is taken to be the directionperpendicular to the sample surface and is controlled by stage 24, asshown above in FIGS. 1 and 2. Changing the Z-coordinate moves the samplesurface in and out of the focus of incident beam 30. The lateral(Y-coordinate) position of the spot is held effectively constant on thewafer, however, in order to avoid changes in peak separation due to thesort of spot displacement that is illustrated in FIG. 7 and explainedabove. As the surface of the wafer moves out of the beam focus (lowervalues of Z in FIG. 10), the apparent separation between peaks 64 and 66increases. The increase is linear with the displacement from the focus(i.e., with Z), as shown by a line 100 that is fitted to the data.

The phenomenon illustrated in FIG. 10 has a number of usefulapplications. For purposes of these applications, the parameters of line100 may be pre-calibrated using one or more samples of knowncharacteristics. For example, the slope and intercept of the line may becalibrated using samples with epitaxial layers having different, knowndopant concentrations and degrees of relaxation. The HRXRD peakseparation at different sample heights may then be measured for a sampleunder test, and the sample characteristics—dopant concentration andrelaxation of the epitaxial layer—may be ascertained based on the slope(and possibly the intercept) of the peak separation as a function of theheight. Alternatively, in order to measure the dopant concentration, thepeak separation may first be measured in symmetric mode (in which theseparation is not sensitive to sample height), and the variation of peakseparation with height in asymmetric mode may then be used to measurerelaxation.

In another embodiment, the sample in asymmetric mode may be positionedintentionally at a height that is out of the focus of incident beam 30in order to increase the separation between peaks 64 and 66. Increasingthe peak separation may be useful in enhancing the visibility of finedetails associated with an epitaxial layer, such as fringe structure 70in the intermediate region between the peaks.

As another alternative, the increased peak separation when sample 22 ismoved out of focus may be used to facilitate placement of beam blocker48 (as shown in FIG. 3A). The beam blocker is positioned precisely toblock substrate peak 64 at the out-of-focus position of the sample, sothat peak 64 is entirely blocked while minimizing blockage of thespectral structure near the peak. The sample may then be moved back intothe focal position, while the position of the beam blocker relative toincident beam 30 remains unchanged. This procedure may be carried outautomatically, under the control of processor 52.

Reciprocal Space Mapping

FIG. 11 is a schematic side view of a system 100 for HRXRD measurement,in accordance with another embodiment of the present invention. System100 is similar in most respects to system 20, and the features shown inFIG. 11 may, optionally, be integrated into system 20. The descriptionthat follows will therefore focus only on the specific elements ofsystem 100 that are used in creating a reciprocal space map (RSM) andthe method of their use for this purpose. RSM is a technique that isknown generally in the art, as described, for example, by Woitok andKarchenko in “Towards Fast Reciprocal Space Mapping,” Advances in X-rayAnalysis 48, pages 165-169 (2005), which is incorporated herein byreference. The embodiment shown in FIG. 11, however, offers advantagesof fine spatial resolution (on the order of 60 μm) and rapid datacollection.

In the present embodiment, a slit 102, oriented in the X-direction,limits converging beam 30 to a narrow range of angles. A scanningmechanism 104, such as a precision motorized drive, scans the slitacross beam 30. Thus, each of a sequence of angles of incidence withinthe angular range of beam 30 is sampled individually, rather thansampling the entire range at once as in the embodiment of FIG. 1. Foreach incident angle, detector assembly 36 senses the diffracted X-rayintensity in beam as a function of angle over the entire range oftakeoff angles that is received by detector array 42.

In this manner, processor 52 collects a three-dimensional (3D) dataset,containing the measured diffraction intensity for each incident/takeoffangle pair. The processor may present these data as a 3D plot, which isknown as a reciprocal space map. This sort of presentation is useful inanalysis of certain types of complex crystalline structures, such aswhen the surface of sample 22 is geometrically distorted.

Although the methods described above relate, for the sake of clarity,specifically to the elements of system and to a certain type of siliconwafer sample and epitaxial layer, the principles of these methods maysimilarly be applied to other types of samples and in other HRXRD systemconfigurations. It will thus be appreciated that the embodimentsdescribed above are cited by way of example, and that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and subcombinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofwhich would occur to persons skilled in the art upon reading theforegoing description and which are not disclosed in the prior art.

1. A method for analysis, comprising: directing a convergingmonochromatic first beam of X-rays generated by an X-ray source at afirst wavelength toward a focus on a surface of a crystalline sample;blocking a second beam generated by the X-ray source at a secondwavelength at a location adjacent to the first beam and before thefocus; sensing the X-rays that are diffracted from the sample whileresolving the sensed X-rays as a function of angle so as to generate adiffraction spectrum of the sample; and analyzing the diffractionspectrum so as to identify a characteristic of the sample.
 2. The methodaccording to claim 1, wherein directing the first beam comprisesfocusing an input X-ray beam using a curved crystal monochromator, whichalso generates the second beam.
 3. Apparatus for analysis, comprising:an X-ray source, which is configured to direct a converging beam ofX-rays toward a surface of a sample having first and second crystallinelayers, with different, respective crystal characteristics, whereby theX-rays are diffracted from the sample so as to generate a diffractionspectrum comprising at least a first diffraction peak due to the firstlayer and a second diffraction peak due to the second layer; a detectorassembly, which is configured to sense the X-rays that are diffractedfrom the sample while resolving the sensed X-rays as a function ofangle; a beam blocker, which is configured to be positioned in theconverging beam so as to block a range of angles in the converging beamthat corresponds to the first diffraction peak; and a processor, whichis coupled to receive and process an output of the detector assembly,while the range of the angles containing the first diffraction peak isblocked, so as to identify a characteristic of at least the second layerbased on the diffraction spectrum.
 4. The apparatus according to claim3, wherein the detector assembly comprises a detector array havingelements configured to capture and resolve the X-rays over a range ofelevation angles simultaneously.
 5. The apparatus according to claim 4,wherein the range is at least 2 degrees.
 6. The apparatus according toclaim 3, wherein the second layer is deposited epitaxially over thefirst layer.
 7. The apparatus according to claim 6, wherein the firstlayer comprises a semiconductor substrate, and the second layercomprises a doped semiconductor.
 8. The apparatus according to claim 7,wherein the semiconductor substrate comprises a silicon wafer, and thesecond layer comprises a SiGe epitaxial layer.
 9. The apparatusaccording to claim 3, wherein the processor is configured to analyze afringe pattern appearing in a vicinity of the first diffraction peak inthe diffraction spectrum.
 10. The apparatus according to claim 3,wherein the converging beam of X-rays has a focus, and wherein theapparatus comprises a motion device, which is configured to shift thesample out of the focus so as to increase a separation between the firstand second diffraction peaks, and wherein the beam blocker is configuredto be adjusted into a position in which the first diffraction peak isblocked while the sample is out of the focus, and to maintain theposition when the sample is shifted into the focus in order to generatethe diffraction spectrum.
 11. The apparatus according to claim 10,wherein the X-ray source and the detector assembly are positionable soas generate the diffraction spectrum in an asymmetric diffraction mode,and wherein the separation between the first and second diffractionpeaks increases as the sample is shifted out of the focus in theasymmetric diffraction mode.
 12. The apparatus according to claim 3,wherein the X-ray source comprises a monochromator, and wherein theconverging beam comprises a monochromatic first beam at a firstwavelength generated by the monochromator, wherein the first beamconverges to a focus on the sample, and wherein the apparatus comprisesa further beam blocker, which is configured to block a second beamgenerated by the monochromator at a second wavelength at a locationadjacent to the first beam and before the focus.
 13. Apparatus foranalysis, comprising: an X-ray source, which is configured to direct aconverging beam of X-rays toward a surface of a sample comprising acrystalline substrate having an epitaxial layer formed thereon, wherebythe X-rays are diffracted from the sample so as to generate adiffraction spectrum comprising a substrate diffraction peak due to thecrystalline substrate and fringes due to the epitaxial layer; a detectorassembly, which is configured to sense the X-rays that are diffractedfrom the sample while resolving the sensed X-rays as a function ofangle; a beam blocker, which is positionable in the converging beam soas to block a range of angles containing a substrate diffraction peak;and a processor, which is coupled to receive and process an output ofthe detector assembly while the beam blocker blocks the range of anglescontaining the substrate diffraction peak and thereby enhances detectionof the fringes at angles adjacent to the range that is blocked, so as tomeasure a relaxation of the epitaxial layer based on a characteristic ofthe fringes.
 14. The apparatus according to claim 13, wherein theprocessor is configured to assess an amplitude of the fringes, wherein areduction in the amplitude is indicative of an increase in therelaxation.
 15. Apparatus for analysis, comprising: an X-ray source,which is configured to direct a converging monochromatic first beam ofX-rays at a first wavelength toward a focus on a surface of acrystalline sample, while directing a second beam at a second wavelengthtoward a location adjacent to the first beam; a beam blocker, which isconfigured to be positioned so as to block the second beam at thelocation adjacent to the first beam and before the focus; a detectorassembly, which is configured to sense the X-rays that are diffractedfrom the sample while resolving the sensed X-rays as a function of angleso as to generate a diffraction spectrum of the crystalline sample; anda processor, which is coupled to receive and process the diffractionspectrum so as to identify a characteristic of the sample.
 16. Theapparatus according to claim 15, wherein the X-ray source comprises anX-ray tube, which is configured to generate an input X-ray beam, and acurved crystal monochromator, which generates the first and secondbeams.
 17. Apparatus for analysis, comprising: an X-ray source, which isconfigured to direct a converging beam of X-rays toward a focus on asurface of a crystalline sample; a slit, which is configured to scanacross the converging beam so as to cause the X-rays in the beam to beincident on the sample at a sequence of angles of incidence over anangular range of the beam; a detector assembly, which is configured tosense the X-rays that are diffracted from the sample at each of theangles of incidence, while resolving the sensed X-rays as a function oftakeoff angle so as to generate diffraction data with respect to each ofthe angles of incidence; and a processor, which is configured to combinethe diffraction data with respect to the angles of incidence over theangular range so as to generate a reciprocal space map of diffractionfrom the sample.